This morning I heard some bluebirds twittering outside the window, so Julia and I took the cue to go out and clean out the nest boxes. The bluebirds’ nest from last year was a dense mass of pine needles and other miscellaneous vegetation, including a single tendril from one of the nearby grapevines (visible below the center in the photo below).
The tree swallow nest on the other side of our front yard vegetable garden was lined with feathers, as is typical for this species. There were some fancy spotted ones that came from the guinea hens that live at a neighbor’s house down the hill and through the woods, several hundred feet away. The ones at the upper edge in the photo below look like barred owl feathers.
Last year was the first time we had a house wren use any of our boxes. House wrens are notorious for filling up multiple boxes with twigs and not necessarily using any of them to nest in. This was the case with both of our backyard boxes last year. Here’s the one from the northwest corner, which in the past has been occupied by chickadees:
When we opened the box at the southwest corner, we were surprised to find the twigs decorated with numerous spider egg sacs:
Now, when I’ve heard people talk about house wren nesting behavior before, I’ve gotten the impression that the wrens just fill up all the boxes because they’re jerks (or, say, trying to crowd out the competition), but this collection of little silken decorations reminded me of the male bowerbirds in Australia and New Guinea, which build elaborate structures in order to attract mates. I checked the Peterson guide to bird nests, and sure enough, it is the male house wren that arrives first and “builds dummy nests of twigs in all or most of available nest sites. Female may or may not accept prechosen site; may or may not accept male’s incomplete twig nest.” If she does accept the twig nest, she adds her own cup of finer material in which the eggs are laid. Nothing in there about the male adding decorations to the twigs, but clearly the one in our yard shared my appreciation for invertebrate architecture. The nest included at least thirteen egg sacs of the common house spider (Theridiidae: Parasteatoda tepidariorum); seven bright yellow egg sacs of pirate spiders (Mimetidae: Mimetus); four tufted egg sacs that I believe belong to cobweb spiders in the genus Euryopis (Theridiidae); four smooth, shining, disc-shaped egg sacs of a hunting (as opposed to web-spinning) spider (Gnaphosidae or maybe Corinnidae); four cocoon bundles of wasps in the genus Cotesia (Braconidae), some of which had a yellow tint suggestive of C. glomerata, a parasitoid of cabbage white caterpillars; three loose, white, spherical egg sacs that I believe belong to the cobweb spider Steatoda triangulosa; three big, fluffy egg sacs that I suspect are from some kind of orbweaver (Araneidae); two cocoons of a spider-parasitizing wasp in the Polysphincta genus group (Ichenumonidae); and one tent caterpillar cocoon (Lasiocampidae: Malacosoma).
Left to right: cocoon of a spider-parasitizing ichneumon wasp; egg sacs of pirate spider and common house spider.
Left, bundle of braconid wasp cocoons; center, egg sac probably of a cobweb spider in the genus Euryopis; lower right, more house spider egg sacs.
Two more pirate spider egg sacs; an egg sac of Steatoda triangularis (bottom right), and in the background, a tent caterpillar cocoon and egg sacs of three other spider species.
A clear view of the tent caterpillar cocoon.
Three hunting spider egg sacs (Gnaphosidae or maybe Corinnidae).
Shortly after we cleared out that box, a couple of chickadees came to check it out. Maybe they’ll beat the wren to it this year.
BugTracks 
All though spider eggs sacs is fascinating!
Fascinating. I wonder how much of this is decoration or could there also be utilitarian twist to this. Wouldn’t new spiderlings offer a convenient home buffet to mama wren while she incubates?
What a fascinating and delightful discovery, Charlie! Seems like they’re doing their best to serve up a gourmet meal in every box. In this spare season, spider egg sacs make sense. Winter wren males also fashion an array of woven cattail leave nests each spring, ahead of female arrival and inspection. Given the plentiful gossamer to be found in marshes much of the year, I’m guessing spider eggs would be served up there too.
What a fun read! I like the idea of putting up a supply of meals for the winter–just like those of us who can fruits and veggies with an eye to future needs.
Wonderful entry! Thanks for the reminder to get on the annual task of nest-box clearing. Watching the house and Carolina wrens vie for the assortment of available cavity nests is a favorite spring activity for me. I felt quite validated by a female house wren a few years back who chose the male’s nest in a gourd I’d grown, cured and hollowed over the fancier wren houses and boxes (they chose the wren box last year for 1st brood) and the plenty of available snags. The assorted paridae in my “patch” will occasionally roost in one of my gourds but tend to prefer a little deeper in the woods , for some reason , and favor snag cavities. I really appreciate the spider sac ID too, you’ve just inadvertently solved an ID mystery for me! Thanks 🙂
I just pulled a house wren nest out of a nest box and counted 26 separate egg sacs/ chambers , mostly spiders I believe. It appears many of them came to be there after the nest was in place based on the spider silk trails visible and the location of most of the sacs being on the upper corners of the nest. It would be neat to try to see how many might be there intentionally from the wrens’ behavior vs. spiders finding a dark hidey place for eggs after the nest was in place.
Hi Charley,
This post about the wren nest with spider egg sacs is really interesting. I have observed the Carolina wrens who live in our yard hunt and eat so many spiders! I get annoyed at them! Maybe placing the egg sacs in the nest is a way to stock their pantry?
Thanks so much for sharing your observations and discoveries.
Best,
Annie Runyon
enjoyed this, especially spider egg casings ID
Very cool! I wish now that I’d examined the egg sacs I find in our wren box every year instead of just chucking the whole pile on the ground.
Super cool!! I’m loving all those spider sacs. Thanks for sharing.
In fact, house wrens have a reason for seeking out twigs with spider egg sacs. The spider eggs will hatch, and will quickly feast on the parasitic mites which tend to feed on the wren nestlings. It is a convenient form of natural pest control in the nest.
I’d love to know if this actually happens. This makes more sense to me than the suggestion some have made that the egg sacs are providing future food for the female or the nestlings–spiderlings are so small that they would be providing a negligible amount of food. In any case, it seems that the female wren was unimpressed by this stockpiling; no actual nesting took place in that box last year.
I had installed 15 bluebird boxes on our farm last year, and a number of them were claimed by the house wrens for their messy, twiggy, unused nests. I emptied them again and again, and finally put up some wren houses for them which they eventually settled into. The chickadees and tree swallows also really like to build in the bluebird boxes… and in the fall and winter, even the mice nest in them.. Two weeks ago, I watched from my kitchen window as a pair of bluebirds were checking out the available real estate. Fingers crossed for another bluebird family this year… I will do my best to provide them with what they need!